1. Field of the Invention
The present invention relates to a seat, and more particularly, to a seat such as a vehicle seat which is installed in a vehicle such as an automobile.
2. Description of the Related Art
There is conventionally known, as a vehicle seat, a vehicle seat having a cushion material formed from polyurethane foam (hereinafter called urethane). As the cushion structure of such a vehicle seat, a structure is widely used in which a cushion material made of urethane is placed on a plate or a spring material, such as a contour mat (trade name) or the like provided at a sitting portion frame or a back portion frame, and these are wrapped in a fabric material.
Therefore, it is known that the shape (the design shape) and the elastic characteristic of the cushion material, and the compressive characteristic of polyurethane foam have a great effect on the dispersability of the body pressure of the seated person and vibration absorbability. Further, by structuring the cushion material by layering urethanes which have various characteristics, a cushion material can be obtained which has a spring characteristic (elastic characteristic) which is close to the spring characteristic of the muscles of the buttocks or the like of the seated person. However, with such a structure, there are the problems that there is the sensation that the restoring force is insufficient and that the weight is heavy.
Thus, a seat has been conceived in which, as the cushion material taking the place of urethane, a cushion structure is structured by stretching, over a frame for a sitting portion, a pair of ground knit fabrics and a two-dimensional knit fabric or a three-dimensional solid knit fabric formed by connecting threads which are disposed between the ground fabrics (see, for example, Japanese Patent No. 5013089 and Japanese Patent Application Laid-Open (JP-A) Nos. 2002-177099, 2002-219985 and 2003-182427). The cushion material formed from this three-dimensional solid knit fabric or two-dimensional fabric is an elastic structure which is difficult to weaken, and is thinner than urethane, and exhibits an elastic characteristic in place of urethane.
However, in a conventional vehicle seat in which such a three-dimensional solid knit fabric or two-dimensional fabric as described above is stretched over the frame for the sitting portion, so-called hip slippage (forward sliding), in which the buttocks of the seated person are moved to the central portion of the seat surface, easily occurs resulting in unstable seating posture.